Typical air compressor systems include an electrical motor for driving a positive displacement machine such as a reciprocating compressor. The electric motor is periodically energized as the pressure in a reservoir for the compressed air falls below a predetermined level and is deenergized once the compressor has increased the air pressure within the reservoir to a different and higher predetermined pressure level. The operating cycle is repeated over a period of time, the frequency of the cycles being dependent upon demand for the compressed air.
Those skilled in the art will readily appreciate that when the electric motor is deenergized as a result of the air pressure in the reservoir attaining the predetermined level, there will be residual air under pressure remaining in the compression chamber of the compressor at a pressure equal to that in the reservoir. Consequently, when pressure in the reservoir has been reduced to the next predetermined level, necessitating starting of the electric motor to once again drive the compressor, the electric motor must start under load as a result of the elevated pressure within the compression chamber of the compressor.
In order to achieve such a start, a larger electric motor than is required to drive the compressor in steady state conditions is required; and that is undesirable because for the vast majority of its operating cycle, such a motor is under utilized and thus only contributes to the expense of the system. To avoid this difficulty, the prior art has resorted to the use of so-called unloading valves. One example in connection with a pressure switch is illustrated in commonly assigned U.S. Pat. No. 3,875,358 issued Apr. 1, 1975 to Dale F. Willcox. The Willcox patent illustrates a pressure switch which operates in response to changes of pressure within the reservoir to alternately energize and deenergize an electric motor for driving the compressor. As is well-known, the pressure switch includes a so-called flipper which changes positions dependent upon whether the switch is opened or closed. In a preferred embodiment, the flipper is utilized to open a small valve which is connected to the compressor downstream of the compression chamber and upstream of the reservoir and a check valve associated therewith. As a consequence, each time the desired pressure level is obtained in the reservoir, the Willcox pressure switch moves its flipper to not only deenergize the electric motor driving the compressor, but to open a small valve connected to the compressor to bleed off to the ambient, any air under pressure that remains in the compression chamber and the conduit connecting the compressor to the reservoir.
As soon as air demand is such as to cause the pressure switch to reenergize the motor, the flipper again changes positions allowing the small valve to close to prevent discharge of compressed air to the ambient. In the meantime, however, the release of air under pressure from the compression chamber of the compressor allows the same to be started at a reduced load thereby allowing the use of a smaller electrical motor than would be required if the unloading valve were not utilized.
While this approach works well in most instances, occasional difficulties arise. In some instances, manufacturers, for cost savings, may attempt to use even smaller motors in the system which, of course, further reduces the starting torque available to initiate operation of the compressor. Even when the motor size is adequate, in instances where the voltage for driving the motor is low (for example when the system is connected to electrical power through a relatively long extension cord) again there may be insufficient torque to properly start the system. In particular, increasing resistance, and thus increasing load upon the motor, begins as soon as the compressor begins to constrict the volume of its compression chamber to compress air and in those instances as identified above, the difficulty may be encountered.
In order to avoid this difficulty, it has been proposed to utilize a normally open valve of relatively large size in lieu of the typical, relatively small, normally closed unloader valve as represented by the above-identified Willcox patent. In this case, the valve is pressure responsive and normally will be open whenever the compressor is started and is operating at reduced speed. That is to say, the valve will remain open so long as the compressor has not been brought up to full speed. As a consequence, the air being compressed by the compressor during start up will be discharged while the compressor is running relatively slowly even though it is accelerating to maintain a low load on the electrical motor throughout the starting process. Once the compressor gets up to speed, it will be operating on a sufficient volume of air so that the resulting pressure applied against the valve is sufficient to close it to prevent further discharge of air to the ambient for unloading purposes.
This system allows the use of smaller motors than those heretofore known, but is not without its own problems In order to be effective during the vast majority of the start up portion of the cycle, the valve must be relatively large and thus the same will have a relatively large pressure responsive surface. As a consequence, when the motor deenergizes, there is a substantial force from residual air under pressure tending to maintain the valve closed; and the pressure switch or other operator that must be utilized to open the valve to release residual gas under pressure to ambient must accordingly be made more powerful to operate successfully against the larger force. Thus, any advantage in reduction of motor size or ability to operate properly under low voltage conditions is partially or wholly offset by the requirement for a more powerful operator to open the valve following deenergization of the electric motor.
The present invention is directed to overcoming one or more of the above problems.